This application claims the priority of German patent document 100 54 33.2, filed Nov. 2, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention concerns a combustion chamber for a rocket engine that expels a stream of hot gases, in particular a cooling device and the inner wall of the combustion chamber that is adjacent to the cooling device.
A combustion-chamber cooling device located adjacent to a combustion chamber is normally intended to keep the combustion-chamber wall, which is heated by the burning gases, cool enough to ensure a satisfactory lifetime for the combustion chamber. However, in special types of rocket engines, such as regenerative cooled engines, it is desirable to achieve enhanced heat transfer to the cooling device because the heat absorbed from the rocket engines by the cooling device is used to increase the engine""s operating efficiency, for example, by running fuel pumps. This means that enhanced heat transfer to the cooling device will result in more efficient engine operation.
One option for enhancing heat transfer in existing technology is described in DE 199 01 422. A disclosed combustion chamber has longitudinal ribs to accommodate a cooling channel containing a coolant. Enhanced heat transfer is achieved by enlarging the inner surface of the combustion chamber. The disadvantages of this approach are the technical difficulties of producing the longitudinal ribs, and the fact that this also increases the engine""s mass. Another option for enlarging the inner surface of the combustion chamber is found in DE 1 135 405 and U.S. Pat. No. 5,221,045, where cooling pipes are formed by curving sections of the internal walls of the combustion chamber upward. This method also involves considerable technical difficulty during production, and it has the additional disadvantages of low structural stability and capacity to withstand stress.
Another conceivable alternative is to lengthen the combustion chamber in order to increase the inner surface area. As the total length of an engine is often fixed, however, this approach can lead to a shortening of the propulsion nozzle, which can result in a reduction in engine performance. Another conceivable measure might be a reduction in the temperature of the combustion-chamber wall on the hot-gas side. With the normal procedures used in cooling devices, however, it is only possible to achieve a slight increase in the difference between the gas temperature in the hot gas stream and the temperature of the combustion-chamber wall, and this would also involve a major loss of pressure in the coolant, which in regenerative cooled engines would also result in a reduction in combustion-chamber pressure.
An object of the present invention is therefore to provide a combustion chamber for a rocket engine that will enable enhanced heat intake by a cooling device in a simple way.
This object has been achieved by providing an inner combustion-chamber wall having depressions that are formed so as to result in flow destabilization of the stable outer layer of the gas stream that forms on the proximity of the inner combustion chamber wall during its operation.
This novel approach provides for a combustion chamber for a rocket engine that expels a stream of hot gases and which has a cooling device. The inner wall of the combustion chamber is the inner surface of the wall between the combustion chamber and the cooling device. In the present invention, the inner wall of the combustion chamber contains depressions. These depressions are formed in such a way that they cause hydrodynamic destabilization of the stable outer layer of the stream of gases that forms in the proximity of the depressions during operation of the combustion chamber.
The present invention therefore takes a different approach to the problem than conventional technology. It can be demonstrated that in the normal case, where there is a smooth inner combustion-chamber wall, gas flow in the combustion chamber in the proximity of the combustion-chamber wall forms a smooth outer layer that develops a certain thermal insulation effect, which works against the combustion-chamber wall absorbing heat from the stream of hot gases. The present invention is configured to systematically disturb the formation of this thermally insulating outer layer, which will considerably increase heat transfer to the combustion chamber wall. This surprising feature of the present invention has a virtually negligible effect on the characteristics of the combustion chamber as a whole because the depressions are formed so that their effect is actually limited to the outer layer.
From page 13 of H. Immich et al.: xe2x80x9cCryogenic Liquid Rocket Engine Technology Developments within the German National Technology Programme,xe2x80x9d AIAA 97-2822, 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Jul. 6-9, 1997, Seattle, Wash., it was known that an increase in heat transfer from hot gases was possible in principle, if the form of the thermally insulating outer layer were disturbed. At that stage of technology, however, no suitable way could be cited by which to successfully achieve this effect. What is more, this document assumed that the production of this effect was technically very difficult and therefore rejected the possibility.
According to the present invention, making depressions with the appropriate form in the combustion-chamber wall has been found to be the solution to the problems cited above, and it provides a simple procedure for eliminating the disadvantages associated with existing technology, as these depressions can be made in the internal combustion chamber wall without great expense.
The depressions according to the present invention should preferably have a maximum depth that is no more than half the thickness of the inner combustion-chamber wall. The depth of the depressions can be adjusted to the expansion of the outer layer in each case. The depressions can therefore have a depth that does not exceed the thickness of the outer layer.
Different types of depressions can be made in order to achieve the desired effects. For example, depressions can be formed by roughening the surface through the use of appropriate blast media. The depressions can also take the form of grooves with a maximum slant of 45xc2x0 against the circumference direction of the combustion chamber. They can also take the form of closed grooves or threading consisting of at least one spiral. Threading is a particularly easy way to make these depressions.
Unless automatically determined by the production method, the form of depressions can be optimized according to different criteria. For example, the cross-section of the depressions can take the form of a segment of a circle, in which the radius of the circle segment is greater than or equal to the depth of the depression. This can reduce the notching-effect of the depressions, which can increase the lifetime of the combustion chamber.
Variation in the density of the number of depressions, particularly in the number of depressions per unit of area in the case of a roughened surface, or the number of depressions per unit of length in the case of grooves, can localize the disturbing effect of the depressions on the outer layer, and in this way adjust local heat transfer for each area of the combustion chamber wall to different conditions and requirements.
It is also currently preferable that different areas of the inner combustion-chamber wall have different densities in the number of depressions. This applies particularly to a combustion chamber where the fuel injection head is found on one end, and on the opposite end, a combustion chamber neck as the exhaust opening for the gas stream. In these combustion chambers, there are particularly high heat-flux values in the area of the combustion-chamber neck.
In order to reduce these values, there can be a higher density in the number of depressions in the area downstream from the combustion chamber neck. In that way, a greater part of the heat can be transferred to the combustion-chamber wall before the heat reaches the nozzle neck. A combustion chamber like that described above also displays relatively low heat flux in the area of the fuel injection head, as the gas flow in this area has a lower temperature than it does further downstream.
Higher density in the number of depressions in this area of the combustion-chamber wall can increase heat transfer to the combustion-chamber wall, so that local heat transfer at this point can be aligned with local heat transfer in areas of the combustion-chamber wall located farther downstream. This makes it possible to achieve even heat transfer over the entire length of the combustion chamber. There can be alternate or cumulative increases in the density of the number of depressions in individual areas of the combustion-chamber wall.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawings.